Cleanup: replace internal option conversion funcs with FSharp.Core ones once shipped

[brianrourkeboll]

Cleanup reminder

#17436 added 'T option ↔ 'T voption conversion functions to FSharp.Core.

When a version of FSharp.Core that contains the new functions ships, the changes from #17457 to replace the internal option conversion functions with the FSharp.Core ones should be applied to this repo.